The business layer is used to enforce business and data rules.The presentation tier uses the services of the business tier.However,the business tier is not tied to any specific client;its services are available to all applications.Business rules can be business algorithms,business policies,legal policies,and so on-for example,"Users get a 10 percent discount for ads placed before Tuesday night" or "Six percent sales tax must be collected for all orders delivered to the Commonwealth of Kentucky."Data rules help to ensure the validity and occasionally the integrity of the stored data-for example,"An order header must have at least one detail record" or "Money must not be lost or generated during trasfers between bank accounts."Business rules are typically implemented in isolated code modules,which are usually stroed in a centralized location so that multiple applications can use them.
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